1. Field of the Invention
The present invention relates to oriented composite polyester films exhibiting, in particular, improved adhesiveness relative to a wide variety of final application topcoatings.
By the expression "final application coatings", as utilized herein, are intended coatings which impart to the oriented polyester base films properties permitting extremely diverse industrial applications, which are well known to this art.
2. Description of the Prior Art
Thus, it is known that, for a number of industrial applications, polyester films cannot be employed directly, but only after the deposition of a topcoating matched to each particular end use. Exemplary of such applications, representative are matte coatings for hand drawing or for tracing tables, photosensitive coatings and photographic applications, diazo coatings for microfilm, printing plans or reprography applications, print coatings, magnetic coatings for recording various data (sound, images, data processing), metallic coatings, and coatings which modify the gas-permeability properties of polyester films used as packaging materials.
It is also known to this art that, normally, the bonding of final application topcoatings to a face surface of a polyester film is not easy. In fact, polyester films have a relatively smooth surface and have limited chemical reactivity and very low sensitivity to solvents. In most cases, therefore, it is necessary to first apply an "adhesion primer" coating, enabling the final use layers to be anchored thereto, according to more or less complex chemical formulations. As a general rule, this adhesion primer (or anchor coat) is applied by the converter. It is thus easy for the latter to adapt the nature of the primer to the end application intended.
Film manufacturers themselves have attempted to modify the surface of the films by various means. The use of polymers of various kinds as adhesion primers has been proposed to the art for this purpose. In most cases, acrylic polymers or copolymers are used, which are preferably employed in latex form, or modified copolyesters are used, particularly copolyesters characterized by the presence of hydrophilic groups and in particular of sulfonyloxy groups (hydroxysulfonyl groups or their metal salts) in their structure. The process of coating polyester films with copolyesters containing sulfonyloxy groups (sulfonated copolyesters) depends on the more or less pronounced hydrophilic nature of the polyester, which itself is related to the sulfonyloxy group content. Thus, when this content is sufficiently high for the copolyester to be soluble or capable of being dispersed in water, the overcoat is produced by coating the polyester film with an aqueous solution or dispersion (cf. French Pat. Nos. 1,401,581 and 1,602,002, U.S. Pat. No. 4,476,189, European Pat. No. 78,559). When the sulfonated copolyester is incapable of being dispersed (not soluble or dispersible) in water, it is preferable to effect deposition by coextrusion of a conventional polyester and a sulfonated copolyester (cf. Japanese Patent Applications published under Nos. 50/134,086 and 54/153,883). The industry is moving towards the use of polyester films with adhesion primers for a wide spectrum of uses and adapted to meet the various needs of the ultimate converters. The production of films of this type has not proven feasible to date using the adhesion primers heretofore known to the art.